1. Field of the Invention
The present invention relates to a polyamic acid varnish composition which is intended to form a polyimide insulating layer of a flexible printed board, and a flexible printed board which employs same.
2. Description of the Related Art
Flexible printed boards in which polyimide insulating layers formed directly onto a metal foil, e.g., copper foil, without an adhesive being interposed therebetween are manufactured by applying a polyamic acid varnish composition, which is obtained through the addition polymerization of an aromatic diamine such as a paraphenylenediamine and an aromatic acid dianhydride such as pyromellitic dianhydride in a solvent such as N-methyl-2-pyrrolidone, onto the copper foil, drying the whole to obtain a polyamic acid layer (polyimide precursor layer), and then heating and imidizing it at 300–400° C. to form a polyimide insulating layer.
However, the presence of carboxyl groups in the polyamic acid gives rise to such problems as the corrosion and discoloration of the copper foil surface when the polyamic acid varnish is applied to the copper foil as described in the foregoing, as well as the formation of copper ions which bring about electrical migration in the flexible printed wiring board.
Accordingly, an imidazole-based rust inhibitor (e.g., Adekastub CDA-1; manufactured by Asahi Denka) is generally added to polyamic acid varnishes which are used upon manufacture of flexible printed boards.
However, conventionally used rust inhibitors dissolve poorly in the polyamic acid varnish, and furthermore, excessive blooming occurs on the polyimide surface under the high temperature heating during imidation, resulting in a deterioration in the adhesive strength of the polyimide with respect to the copper foil. Moreover, the rust-inhibitor is scattered during imidation, forming a vapor residue substance that deposits on the imidation process apparatus and on the flexible printed board products and contaminates them.